1. Field of the Invention
The present invention relates to a circuit for automatically adjusting the background threshold of a reading signal for a digital image scanning device, comprising means for scanning the pixels, capable of generating corresponding electrical reading signals and transmitting them in series to the adjusting circuit and memory means controlled by the regulating circuit for storing digital values of the signal in binary code in dependence on the threshold, the adjusting circuit defining the threshold (V.sub.s) of the signal in dependence on the positive-peak (V.sub.max) and negative-peak (V.sub.min) signals previously received from the scanning means.
2. Description of the Related Art.
In apparatus of the above indicated type wherein a signal produced by a photoelectric detector constituted of an array of image sensing elements such as a so called CCD device (Charge Coupled Device), an analog output signal produced by such a device must be compared against at least one threshold in order to derive therefrom a binary-type signal.
In order to obtain good performances from a scanning apparatus of this kind for scanning like office documents, it is necessary to adjust the level of the threshold which determines the digital conversion of the above said analog signal.
Several circuitries have been developed for rendering adaptive the level of the threshold in order to optimize the operation of the system.
For instance, the U.S. Pat. No. 4,251,837 discloses a circuit of the above-indicated type, which includes a switching circuit controlled by two comparators for comparing the threshold signals, in order to select three different thresholds. However that circuit is not capable of adapting the threshold to the type of image or to the background colour of the image.
U.S. Pat. No. 4,554,594 of the present assignee also discloses a variable threshold circuit for serial scanning and digital processing of images with various grey levels, in which a given threshold is selected only if it maintains a certain persistency defined by a counter. However that circuit is also not capable of adapting the threshold to the "background colour" of the image.